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The Philippine Sea Plate (PSP) experienced repetitive arc activities and back-arc
spreading or rifting in intra-oceanic environments, at youngest since the Early
Cretaceous (Fig. 1 ). Early to Late Cretaceous arc igneous rocks are known to occur
in the Amami Plateau (Hickey-Vergas 2005 ). The plateau, together with the adjacent
Daito and Oki-Daito Ridges, is considered as a Cretaceous remnant arc left after
back-arc spreading of the West Philippine Basin (WPB) during the Paleocene to the
Eocene (Hilde and Lee 1984 ). The correspondent daughter arcs have been assumed
to be Cretaceous ophiolites and arc sequences exposed on the Halmahera and eastern
Mindanao islands (Hall et al. 1995 ; 1988 ). As WPB spread and the PSP rotated, the
proto-Izu-Bonin-Mariana (IBM) arc formed at the present-day Kyushu-Palau Ridge
at ~49 Ma (Ishizuka et al. 2006 ). The IBM arc experienced back-arc rifting and
spreading in the late Oligocene to middle Miocene (the Shikoku and Parece Vela
Basin: Okino et al. 1994 ), in the late Miocene (between the Nishi-Shichito Ridge and
the present-day IBM arc: Yamazaki and Yuasa 1998 ; Kodaira et al. 2008 ), since the
Pliocene (West Mariana Basin: Husson and Uyeda 1980 ), and in the Quaternary
(intra-arc rift basins in the Izu-Bonin Arc). Since the Oligocene, the modes and direc-
tions of the back-arc spreading separated between the Izu-Bonin and Mariana arcs
bounded by the Sofugan Tectonic Line (STL: Yuasa 1985 ). The Ohmachi Seamount
is located at the northeastern termination of the STL and the Nishinoshima Trough,
where the IBM arc crust is extremely thin (less than 10 km: Kodaira et al. 2007 ).
There have been several ideas proposed for the exhumation of the serpentinite body
in the Ohmachi Seamount. The earliest model considered it as a diapiric body uprose
along with the Quaternary intra-arc rifting (Yuasa et al. 1999 ). Ohara ( 2003 ) also
attributed the serpentinite exhumation to the Quaternary rifting, but considered it as an
oceanic core complex similar to the other core complexes being discovered in the
back-arc spreading systems of the PSP (Ohara et al. 2001 , 2003 ). Ueda et al. ( 2004 )
assumed that the timing of the exhumation was older than the overlying volcanic rocks.
They first concerned the occurrences of high-pressure metamorphic rocks, and attrib-
uted their exhumation to extension by the spreading of the WPB over the subducting
Pacific slab in the Eocene. Meschede et al. ( 2009 ) proposed that the unroofing resulted
from leaky transform displacement (transtension) along the STL based on the differ-
ence of spreading directions between the Shikoku and Parece-Vela basins. Most
recently, Ota and Kaneko ( 2010 ) interpreted the base of the Tertiary as a post-Miocene
fault, and attributed the exhumation to squeezing by the shallowing Pacific slab related
to subduction of a large buoyant seamount (Ogasawara Plateau: Fig. 1 ).
Recently accumulated geological and structural observations suggest that the ser-
pentinite body was emplaced earlier than the deposition (or basal faulting) of the
overlying Paleogene formation, which is scarcely deformed, and whose base crosscut
the foliations of the serpentinite body. The sub-horizontal stretching lineation and
subsequent lineation-orthogonal fractures are not consistent with diapiric movements,
which prefer dip-slip strain. Hence, diapirism is unlikely to have been the main mode
of the exhumation, although buoyancy and volume expansion could have contrib-
uted to upward components of the displacement. The NNE-trending stretching lin-
eations are compatible neither with spreading direction of the Quaternary intra-arc
rift nor the present-day moving direction of the subducting Pacific plate (Fig. 1 ).
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